1. Field of the Invention
The present invention relates to a power supply, and more particularly to a power supply with surge voltage control functions minimizing a power supply loss.
2. Description of the Related Art
In general, AC power sources provide a voltage of 110V or 220V. Korea and Europe uses AC power sources supplying a voltage of 220V, whereas United States of America and Japan use AC power sources supplying a voltage of 110V. Accordingly, a power supply built into electronic devices is generally provided with a voltage selection switch to select 110V or 220V as an input voltage thereto.
In the meantime, on occasions when an electronic device normally used in an area with the AC power source of 110V is used in an area with 220V, users usually forget about manipulating the voltage selection switch provided on the power supply, which often causes severe damage to the power supply due to an excessive voltage supplied thereto, or causes the power supply to output unstable voltages or operate in a malfunctioning state due to shocks applied thereto even though the power supply is not broken down.
FIG. 1 shows a detailed circuit of a conventional power supply.
The power supply shown in FIG. 1 has a fuse 10 connected in series to an AC input terminal AC1, an LC filter 20 connected to the fuse 10 and another AC input terminal AC2, and a rectifier 30 to rectify an output voltage of the LC filter 20.
The fuse 10 is electrically open-circuited when an AC input source supplies a voltage over a certain potential level, to thereby protect the power supply. The power supply usually has voltage characteristics to withstand 250V in countries supplying 220V through the AC power source, and also has diverse current capacities depending upon electronic devices in which it is built.
The LC filter 20 eliminates noise included in the AC power source. The frequency of the AC power source ranges from 50 Hz to 60 Hz, and the LC filter 20 eliminates frequencies lower or higher than this frequency range.
The rectifier 30 rectifies and converts into a dc voltage a voltage of the AC power source filtered through the LC filter 20. The rectifier 30 full-wave-rectifies, through a bridge diode 31, a voltage of the AC power source that is temperature-compensated by a thermistor 33, smoothes the rectified voltage through a capacitor 32, and converts the smoothed voltage into a complete dc voltage. At this time, an electrolytic capacitor having excellent low-frequency characteristics is usually used for the capacitor 32.
The thermistor 33 is an element that lowers its resistance value as an ambient temperature rises, which limits an electric current that is applied to the bridge diode 31 before the power supply reaches a steady state. If the power supply reaches the steady state, the power supply usually radiates a certain amount of heat, and the resistance value of the thermistor 33 is set to have an optimum value when the power supply has a predetermined temperature in the steady state. Accordingly, before the power supply reaches the steady state, excessive current is prevented from flowing into the bridge diode 31.
In the meantime, the electrolytic capacitor 32 is a capacitor formed with a metal film and a dielectric layer inserted in electrolyte, and explodes if a voltage outputted from the rectifier 30 exceeds the voltage that the power supply can withstand, and, when exploded, the electrolyte splashes over the circuit components constituting the power supply.
Since the electrolyte is electrically conductive, the splashed electrolyte short-circuits neighboring circuit components, thereby breaking down the power supply. Further, a dc voltage smoothed through the electrolytic capacitor 32 applies stress on the power supply just before the electrolytic capacitor 32 explodes, which deteriorates electric characteristics of the circuit components constituting the power supply.
FIG. 2 is a circuit diagram showing a power supply protection device formed at an input stage of another conventional power supply.
The power supply protection device shown in FIG. 2 has a varistor 50 connected to the AC input terminals AC1 and AC2, a fuse 40 connected in series to the AC input terminal AC1, an LC filter 60 connected in series and in parallel with the fuse 40 and the AC input terminal AC2, respectively, to eliminate noise included in a voltage of the AC power source.
The varistor 50 is an element that lowers its internal resistance value as a value of the voltage applied thereto increases, which forms a current path between the AC input terminals AC1 and AC2 when a potential difference therebetween increases so that an excessive voltage is not applied to the LC filter 60. In general, the varistor 50 consists of a ZnO2 material of conductivity, and a surge current passes therethrough when a voltage applied thereto exceeds a predetermined voltage. At this time, if the surge current exceeds the limit capacity of the varistor 50, the varistor 50 is broken down so that the power supply can not be protected, and fragments thereof are scattered, thereby impacting on its ambient components. Further, there is a problem in that the scattered fragments cause the electric parts of the power supply to be short-circuited so that additional damage is incurred by the parts.